This type of sensor is used e.g. to examine the function of the liver of an examinee. In this examination, a specific pigment which is selectively taken into and excreted from only the liver is injected into the blood stream. Then the concentration of the pigment in blood plasma is measured with the sensor to determine the blood plasma disappearance rate and retention ratio of the pigment to examine the function of the liver.
As one example of such prior art photosensors, there is known a bio-photosensor which is applied to a finger and in which light is transmitted from one side of the finger to the other side so as to detect the fluctuation of transmittance or reflectance of light due to the blood flow. Signals representative of such fluctuations are processed and calculated to obtain such vital information as pulse rates or blood pressures (see Japanese Unexamined Patent Publication No. 60-34432 and Japanese Unexamined Utility Model Publication No. 60-158803).
FIGS. 1A and 1B show a prior art bio-photosensor comprising a strip of flexible base film 1, and a light emitter 2 and a light receptor 3 mounted on the base film 1 and spaced longitudinally from each other. The emitter 2 and the receptor 3 are connected respectively to lead wires 4. An adhesive tape 5 is used to fix the photosensor 6 to a finger.
The base film 1 provided with the emitter 2 and the receptor 3 is applied to a finger 7 in its longitudinal direction and turned back at the finger tip 7a. (FIG. 1C) The tape 5 is then put over the base film 1 to fix the photosensor 6 to the finger 7.
In this state, the sensor 6 is turned on to cause the light emitter 2 to give forth light which passes through the finger 7 and strikes the receptor 3. In response thereto, the receptor 3 applies signals to a signal processor (not shown) through the lead wire 4 and a connector (not shown). The processor will process the signals about the fluctuation of light transmittance to obtain vital information such as pulse rate and blood pressure.
In this prior art photosensor, wires are directly connected to the emitter and the receptor and they have to be fixed to the flexible base film. Means and step of fixing the wires to the base film will add to the cost of the photosensor.
Also, the rigidity of lead wires extending in the longitudinal direction of the base film will lower the flexibility of the base film. Such a photosensor could not be comfortably fitted on part of the body of an examinee. Further, there was a fear that the wire or cable might be pulled out of the sensor or be broken at their connections during use only with a small tensile or bending stress which tends to act on the connections.
In FIG. 1C, the emitter 2 and the receptor 3 are set in exactly opposite positions to each other. But if they are not, it is necessary to increase the power of the emitter 2 or the amplifying factor of the receptor 3. If the power of the emitter is increased, the finger might be burned due to a heat buildup of the emitter. An increase in the amplifying factor of the receptor would impair the S/N ratio, thus lowering the accuracy of the photosensor. This problem results from +he fact that the photosensor is not equipped with means for ensuring the relative position between the emitter 2 and the receptor 3 when the photosensor is fitted on the finger.
Such a prior art photosensor is provided on its base film with a light emitter capable of emitting two beams having different wavelengths from each other. However, no photosensor is equipped with a light emitter capable of emitting two beams, one cf the beams having such a wavelength as to be absorbed almost entirely by a specific pigment which is removed exclusively by the hepatic parenchyma once it is injected into the blood and the other having such a wavelength as not to be absorbed by the abovesaid pigment. Elaborate equipment was therefore necessary to measure the absorption of a pigment e.g. in the blood.